Bacillus sphaericus is a sporeforming bacterium with a high degree of potential as a microbial control agent for vector mosquitoes. This organism kills the host by means of a protein toxin. The toxin is highly insecticidal to larvae and cultured cells from many species of mosquitoes, but is much less toxic to certain mosquito species such as Aedes aegypti and is not toxic to insects other than mosquitoes nor to mammals. This proposal describes experiments that seek to examine the binding of this toxin to cultured and mosquito larval cells and to explore the mode of action both in vitro and in vivo. Resistant mosquito species will be compared to sensitive species, to determine the cause of reduced sensitivity. Labelled toxin will be used to study binding and internalization of the toxin. The component of the cell membrane responsible for binding in sensitive species will be examined, isolated, and antiserum raised to this receptor will be used to examine proteins of resistant species. Monoclonal antibodies will be raised to the toxin and the receptor, and will be used to investigate binding and mode of action of the toxin. Possible modes of action including development of membrane pores and changes in cyclic-AMP will be explored. An ultrastructural study will seek changes in the cell membrane of cultured and larval midgut cells associated with intoxication by the B. sphaericus toxin, and trace movement of gold-labelled toxin in cultured cells and in the larvae.